As a conventional apparatus for manufacturing light-emitting elements, there is an in-line successive deposition apparatus (e.g. Patent Document 1). The in-line successive deposition apparatus has a plurality of deposition chambers which are arranged and coupled in a line and a transfer unit for transferring a substrate in the plurality of deposition chambers. Each deposition chamber has a plurality of linear deposition sources arranged in a line in a direction in which the substrate is transferred, and respective longer directions of the linear deposition sources are perpendicular to the substrate transfer direction and parallel to each other. With such an in-line successive deposition apparatus, an EL layer can be consecutively formed over an anode over a substrate with the plurality of deposition sources while transferring the substrate in the plurality of deposition chambers. The EL layer at least includes a light-emitting layer, and may further include a hole-injection layer, a hole-transport layer, an electron-transport layer, an electron-injection layer, etc. stacked as appropriate.
As for the structure of the light-emitting layer, a structure in which a dopant material which is a light-emitting substance is mixed with a host material is preferably employed to improve the luminous efficiency. As the host material, it is preferable to use a host material whose highest occupied molecular orbital level (hereinafter referred to as “HOMO level”) is lower than that of the dopant material (or a light-emitting substance which emits visible light). The properties of the dopant material differ form those of the host material; thus, respective deposition sources need to be separately controlled in order to adjust the mixture ratio to deposit the light-emitting layer. In view of the above, in the above-descried in-line successive deposition apparatus, the linear deposition source of the dopant material and the linear deposition source of the host material are alternately disposed, so that the host material and the dopant material from respective linear deposition sources adjacent to each other are mixed to be deposited.
As the material of the anode, a metal, a metal oxide, or the like whose conductive property is higher than that of an organic compound is preferably used. This is because power loss can be suppressed. However, the work function of the metal, the metal oxide, or the like is higher than the HOMO level of any material of the light-emitting layer, and a difference therebetween makes it difficult to inject holes from the anode to any material of the light-emitting layer.
Hence, a structure is known in which a hole-transport layer is formed using a material whose HOMO level is lower than the work function of the anode and is higher than the HOMO level of any material of the light-emitting layer between the anode and the light-emitting layer to attenuate the difference between the work function of the anode and the HOMO level of any material of the light-emitting layer. Owing to the hole-transport layer, holes are transported readily via a material whose HOMO level is relatively high (e.g., light-emitting substance) of the light-emitting layer from the anode to the light-emitting layer.
However, in the in-line successive deposition apparatus, a single layer of the light-emitting substance, which is the dopant material, or a single layer of the host material whose HOMO level is the lowest in the materials of the light-emitting layer tends to be formed in, the interface between the hole-transport layer and the light-emitting layer. The single layer of the light-emitting substance leads to occurrence of concentration quenching of the light-emitting substance. Further, if the difference between the HOMO level of the material of the hole-transport layer and the HOMO level of the host material is large, the single layer of the host material leads to a reduction in hole transport from the hole-transport layer to the light-emitting layer. Accordingly, formation of the single layer causes an increase in the driving voltage of the light-emitting element or a reduction in luminous efficiency. In this specification, the “single layer” is a layer formed of a single material.
Particularly in the mass production of light-emitting elements, which requires efficient manufacturing, the deposition rate of the linear deposition source and the transferring speed of the substrate are set to high, whereby deposition is performed at high speed. However, such high speed deposition has a problem in that the above-described single layer is more likely to be formed in the interface between the hole-transport layer and the light-emitting layer.
[Reference]
    Patent Document 1: Japanese Published Patent Application No. 2006-261109